We examined maternal transfer efficiency, retention by subsequent generations, and transgenerational toxicity of methylmercury (CH3Hg or MeHg) in a population of freshwater zooplankton (Daphnia magna). The effect of dietary MeHg residence time in the daphnids on the efflux system also was quantified. After ingesting a relatively high dosage of MeHg, D. magna exhibited a reduction of live neonates and an increase of undeveloped eggs (or embryos), which reflected the sublethal toxicity of MeHg. The daily maternal transfer efficiency of MeHg to both reproductive outputs ranged from 0.42 to 4.9% over different ages of the parental daphnids, which was dependent on the daily reproductive output. During the lifetime of D. magna, reproduction contributed to 10.8% +/- 1.74% (n = 3) SD of total MeHg loss from the parental daphnids. The percentage of MeHg retention by the second generation (F1) of D. magna (40-60%) was generally higher than that by the parental generation (F0; approximately 25%) after 20 d of depuration. Methylmercury imposed sublethal toxicity to the F0 and F1 generations, but a smaller effect was observed on the F2 generation. Because of the very low MeHg body burden in the subsequent generations, we hypothesized that factors other than MeHg, such as nutritional deficiency in the offspring contributed to the transgenerational toxicity. Different MeHg residence times did not significantly affect the efflux rate of MeHg but did significantly affect the relative importance of reproduction as the elimination pathway for MeHg. Based on the MeHg body burden of neonates, we estimated that MeHg took 2.5 to 3.0 d to be optimally transferred from assimilation (e.g., gut) to the site of egg development (e.g., brood chamber) in D. magna. Our study demonstrated that maternal transfer of MeHg in freshwater zooplankton is an important predictor of MeHg concentration in their offspring and is a time-dependent and highly dynamic process.